91 infiniti G20. running great not using oil 70,000 miles. Yestermorning turns over will not start. Tow to shop. compression

0,40,25,60. Add oil to cylinder, compression 50,60,30,80. Starts. drive 5 miles. compression 100,100,100,100. Zero to 100 doesn’t make sense. Any explanation and do I need a new engine. Runs great right now.

I don’t know what to say about those compression readings except someone’s compression tester may not be working correctly (leaking past the Schrader valve).
Even those 100 PSI readings are garbage and I don’t see how this car could run “great” with readings that low.

The general rule of thumb for compression figures (varies a bit by altitude, barometric pressure, etc.) is about 18-20 X the compression ratio.
Compression ratio is about 9.5 to 1 so this means a good, normal engine should produce pressures in the 175-185 PSI range (again, dependent on conditions)

I don’t see a 70k miles Infiniti as needing an engine unless it’s been seriously overheated in the past and even then it would not run great if those figures are legitimate. Another test by someone else with a different tester might be worth doing.

(And I would be extremely surprised if this engine would even start with 100 PSI much less run great. With the 30-80 PSI figures I don’t see it starting at all.)

I would be tempted to blame this on a conincidence. Since the car is 17 years old maybe a failing ignition switch leading to loss of power to the coil perhaps???

No idea what’s behind those compression figures but no way is an engine that averages 50 or so PSI going to start, much less run great.

I mention the compression tester failure because I’ve had this happen to me a couple of times. It’s also why if a problem appears to crop up in regards to compression I then go back and perform the test again with a different tester; and sometimes twice more with 2 testers. I hate guesswork.

What I’ve seen with testers sometimes is that a Schrader valve can leak a little (just like a tire valve stem) or a near microscopic piece of dirt or carbon will get blown into the tester and wedge in the Schrader valve seat. Depending on the severity of either of these compression readings can be low; or in one instance I was involved in all I got were zeros across the board.

The cylinder will force air into the tester on the upstroke and then suck most or all of the air right back out of the tester.